Generally, optical filters and coatings are passive components whose basic function is to define or improve the performance of optical systems. There are many types of optical filters and they are used for a broad range of different applications. One common type of optical filter is a sunglass lens. Polarized sunglass lenses filter out light with a certain direction of polarization in addition to reducing the sun's intensity. Applications of optical filters and coatings can be diverse as in anti-glare computer screens, colored glass, sighting devices, and electrical spark imagers—to name just a few.
Some optical filters are specialized for different wavelength ranges of light because of limitations in available materials that are optically transparent in the range of interest. For example, many applications and instruments require optical filters that can be used to tune the optical behavior of light in the near infrared, mid infrared or far infrared wavelength range (i.e., at frequencies of radiant energy that are generally below the frequencies of visible light). Some example applications for such filters include far- and mid-IR focal-plane arrays for military applications, chemical sensing, astronomy, wavelength division multiplexing in optical communications, space observations to name a few.
Much work has been done in the past to develop useful optical filters and coatings for different wavelength ranges. Widely spread filter types include: absorption-based filters (i.e., filters where the rejection of light is caused by absorption in filter material) and interference-based filters (i.e., filters where the rejection of light is caused by reflectance from multiple layers composing the filter). Detailed discussion of such filters can be found in for example Macleod H. A., Thin-Film Optical Filters, 3rd ed., Institute of Physics Publishing, 2001.
Exemplary Absorption Type Filters
Absorption filters generally consist of a thin film or slide of material that has an absorption feature (band or edge) at the required wavelength or incorporates an optically excitable material, such as a color center. Filters that utilize semiconductor material can serve as an illustrative example of an edge absorption filter (more particularly long-pass filters). Semiconductors are known to have an absorption band that extends to some characteristic wavelength, which corresponds to the bandgap energy of a particular semiconductor. The transmission/rejection edge could be made very sharp for a semiconductor layer thicknesses above 100 μm. Absorption band edge of different semiconductors and semiconductor composites can vary from ˜500 nm for gallium phosphate (GaP) and aluminum arsenate (AlAs) to more than 2 μm for indium arsenate (InAs) and InSb. The absorption band edge can be smoothly tuned by adjusting the semiconductor composition (for example, AlxGa1-xAs absorption band edge tunes quite linearly from 2.1 eV for x=1 to 1.4 for x=0), long-pass filters can be obtained with a reasonably sharp edge for any wavelength at the ˜500 to 2400 nm range). However, at least some semiconductor-based long-pass filters have a significant disadvantage—i.e., high reflection losses caused by the high refractive index of semiconductor materials. Such a problem is usually solved by antireflection coatings of both semiconductor surfaces in the case of semiconductor wafer (or slide) used or, in the case of thin film semiconductor material, of the top and between the substrate and the layer of semiconductor material. However, such an approach can also have some significant disadvantages. While the absorption edge shape and position of uncoated semiconductor film does not depend on angle of incidence (only the degree of an absorption value changes), both the absorption edge shape and position of antireflection-coated semiconductor film depend on angle of incidence. Hence, the filter can in at least some cases be used effectively only for some limited angular range. In addition, semiconductor absorption edges show strong temperature dependence. It also should be noted that semiconductor-based absorption filter cannot be used as a band-pass or narrow-band pass filtering, since the absorption bands of semiconductors are generally wide.
Other materials that are sometimes used to form absorption filters include colored-glass filters (for example, Schott glass filter). Such colored-glass filters generally operate through the process of ionic absorption of inorganic material, dispersed uniformly through the glass slide or through the absorptive scattering of crystallites formed within the glass. Such filters generally offer fairly wide design freedom in terms of absorption band position and can be arranged either in short-pass, long-pass or band-blocking forms. The significantly lower refractive index of such filters as compared to semiconductor filters make the reflection losses lower. The temperature dependence of the rejection band edge position of such filters is also lower than that of semiconductors. However, such filters can also suffer from considerable limitations. For example, the transmission through the transparency range of such filters is usually not very uniform therefore, such filters in some cases cannot tolerate high power and/or are not well-suited for narrow bandpass filters.
Exemplary Interference Type Filters
Generally, the basis of interference filters is a Fabry-Perot interferometer. A Fabry-Perot (FP) interferometer can be imagined as a thin film having two flat surfaces that are parallel to each other and coated with relatively high-reflectance coatings. In practice, a different realization of Fabry-Perot interferometer is common—the so-called Fabry-Perot etalon, which consists of two flat plates separated by a distance d and aligned parallel to each other with a high degree of accuracy. The separation is usually maintained by a spacer ring made of quartz or Invar, and the inner surfaces of the two plates are usually coated to enhance their reflections. The spectral dependence of the transmittance through an FP interferometer contains peaks and valleys. The peaks of transmittance are known as fringes. It can be proven that the maximum values of transmittance at the peaks reach unity if the Fabry-Perot interferometer is constructed from nonabsorptive materials.
The classic design of a high-reflectance coating is based on alternating quarter-wave layers of two different materials. The high reflectance in a quarter-wave layer stack takes place because the light beams, reflected from all the interfaces in the multilayer, are in phase when they reach the front surface where constructive interference of all the reflected waves occurs. As with the Fabry-Perot etalon, the reflectance spectrum of such a multilayer contains multiple reflection peaks corresponding to the phase-matching conditions of the reflected waves at the different interfaces. However, these reflection maximums are considerably wider than that of the Fabry-Perot etalon. The width of the high reflectance plateau depends on the refractive index contrast between the low and high refractive index materials that compose the high reflectance multilayer. As follows from the above discussion, the high-reflectance dielectric multilayer can be constructed to have very high reflectance over a wide range of wavelengths. However, such multilayers can have some disadvantages. One possible disadvantage is that the high reflectance zone of such a reflector, although it can be made wide, is still limited. Moreover, since the reflectance peaks are located where the waves reflected from each interface in multilayer are in phase, the wavelength positions for the reflectance peaks may strongly depend on the angle of incidence, similar to the Fabry-Perot etalon case.
A quarter-wave stack can be considered as the basic type of interference edge filter. The transmission spectrum of the quarter-wave stack contains alternating low- and high-reflectance zones and, hence, alternative high- and low-transmittance zones. Such a filter can be used as a long-pass filter or as a short-pass filter. Edge wavelengths can be tuned by changing the wavelength at which the stack is quarter-wave. Such an edge filter will be suitable for relatively narrow-band applications, that is, when the width of the rejection zone is greater than the spectral width of light to be eliminated. For all other cases, the required elimination of all wavelengths shorter than (or longer than) a particular value requires a different filter design.
Perhaps the simplest design of a narrowband-pass filter is the Fabry-Perot filter discussed previously. However, the spectral pass-band shape of the Fabry-Perot filter is triangular. In addition, the original design generally may require two precisely aligned and spaced high-flatness plates, which may not be practical for many applications. Therefore, narrowband-pass filters are usually made in slightly modified form with respect to a Fabry-Perot etalon. A Fabry-Perot thin film filter is a thin film assembly consisting of a dielectric layer bounded by either two metallic reflecting layers or by two multilayer dielectric reflectors. The realization of a Fabry-Perot filter with metallic reflective layers is called a metal-dielectric Fabry-Perot filter, while a Fabry-Perot filter with two dielectric multilayer reflectors is called an all-dielectric Fabry-Perot filter.
The metal-dielectric Fabry-Perot filter is perhaps the simplest realization of a narrowband-pass filter. In such filters, the dielectric layer, surrounded by metal reflection layers, serves as a spacer in the Fabry-Perot etalon. and therefore, is called a spacer layer. The metallic reflective layers must provide reasonably high reflectivity at the surface while keeping losses as low as possible. For the visible region of the spectrum, silver is an optimal metal, while for ultraviolet and deep ultraviolet aluminum is the preferred material. However, other metals can be used as well.
The degree of light absorption is perhaps the biggest disadvantage of metal-dielectric narrowband-pass filters. Although absorption during a single reflection from thin metal film is small and absorption during transmission also can be minimized by using thin metal films, in a Fabry-Perot cavity absorption is greatly enhanced due to multiple reflections of the transmitted light. Metal-dielectric Fabry-Perot filters have the same dependence of the transmittance peak wavelength as the Fabry-Perot cavity. In addition to this disadvantage, the absorbance of such a filter strongly depends on the angle of incidence. Such filters are usually used in applications where other filters, such as all-dielectric Fabry-Perot filters, are prohibited by either cost or other factors, such as the inability to function in the deep
As was discussed above, in the all-dielectric Fabry-Perot filter the metallic reflecting layers are replaced by high-reflectance dielectric multilayers. Two different cases of such filters can be considered: {Air| H L H L . . . H L H H L H . . . L H L H| Substrate} and {Air| H L H L . . . L H L L H L . . . L H L H| Substrate}, where H indicates the higher index of refraction and L indicates the lower index of refraction. The refractive indices of the layers adjacent to air and the substrate should be high to maximize the reflection from the multilayer. The transmission spectrum of an all-dielectric Fabry-Perot will be a narrow maximum within a broad minimum. The width of the maximum and the transmittance at the maximum will depend upon the reflectivities of the two multilayer stacks.
The central position of the transmittance peak in the all-dielectric Fabry-Perot filters is generally the same as for air cavity Fabry-Perot filters. However, the effect of variations in angle of incidence can be more severe for narrowband-pass filters than that of bandpass, band edge or multilayer reflectors due to a generally narrow transmittance peak. Shifts of the central position of the transmittance peak as strong as 800% in terms of the transmittance peak half width at just a 30 degrees tilt are not uncommon. Such a strong angular dependence of the transmittance spectra of all-dielectric Fabry-Perot filters causes strong dependence of the transmittance spectra on the convergence (or divergence) of the incident beam. Hence, all-dielectric Fabry-Perot filters are often suitable only for plane-parallel or slightly convergent or divergent beams, which causes additional complexity in the optical designs employing such filters.
As was discussed above, the transmitted spectral shape of the all-dielectric Fabry-Perot filter is generally not ideal. For many filter purposes, a nearly rectangular shape of the transmittance spectra is desired. In addition, the maximum achievable rejection in the rejection zone of the filter and the bandwidth of the transmission zone are related. That is, for a given rejection factor, the bandwidth value of the filter is predetermined if the refractive indices of the layers in the filter are fixed. The solution of this problem was found in using multiple-cavity filter designs.
Exemplary Multi-Cavity Interference Filter Designs
Perhaps the simplest type of multiple cavity filter is a double-cavity filter. Such a filter has the structure of {Air|reflector|half-wave spacer|reflector|half-wave spacer|reflector|Substrate}. Such a structure can have some advantages with respect to a single-cavity design. However, for some applications such as dense wavelength division multiplexing (DWDM), a better spectral shape may be needed. The important criteria in high-performance, narrowband-pass filters are steeper edges and a flatter top on the transmission peak. For two-cavity filter designs, the peaks at both sides of pass band (so-called “rabbit's ears”) are prominent. In this case the number of cavities needed can be considerably more than two to reduce the “rabbit's ears.”
Although at normal incidence the advantages of multiple-cavity filters are geneally strong, the effects of variations of angle of incidence and beam divergence on the transmittance spectra can be more severe for multiple-cavity, all-dielectric Fabry-Perot filters than for single cavity filters. This occurs because the rectangular shape of the pass band of the multiple-cavity filter is due to phase matching between the light waves reflected from the different reflector stacks in the multiple-cavity structure. The phase-matched conditions hold only for a distinct angle and wavelength. Unlike single-cavity filters, where the transmittance peak generally experiences a wavelength shift when illuminated at non-normal angles without significant perturbation of its shape, in multiple-cavity, all-dielectric Fabry-Perot filters the shape of the transmittance band generally changes dramatically with variations in the angle of incidence. The flat top of the multiple-cavity filter at normal incidence frequently resolves into separate narrow transmittance peaks related to the interference between the waves reflected from different reflector stacks within the multiple-cavity multilayer structure. Hence, multiple cavity, all-dielectric Fabry-Perot filters can become unusable at incident angles more than 3 to 5 degrees from normal incidence. Such a property is important in DWDM filters where several hundred layers may be required to produce flat topped transmittance bands with bandwidths narrower than 1 nm. Additional precise mechanical alignment may solve this problem, but with additional complexity and resultant additional cost.
In addition, multiple-cavity, all-dielectric Fabry-Perot filters generally require the incident beam to be highly collimated. The shape of the pass band of such filters degrades significantly even for Gaussian beams, the convergence (or divergence) angle of which is about 10-15 degrees. Hence, multiple-cavity, all-dielectric Fabry-Perot filters generally can require not only precise mechanical alignment to ensure normal incidence of the beam, but also a high degree of collimation. Several other significant disadvantages exist with multiple-cavity, all-dielectric Fabry-Perot filters. These disadvantages may include the presence of long-wave pass bands (i.e., wavelength-limited rejection bands) and significant difficulties in manufacturing such filters for short wavelength spectral ranges (deep and far ultraviolet). For the applications that require the useful filter properties in the UV ranges, multiple-cavity metal dielectric filters are usually used. In particular, it has been found that, in addition to the disadvantages of an all-dielectric Fabry-Perot filter such as the relationship between pass-band bandwidth and maximum obtainable rejection and the resultant triangular shape of the pass band, the single-cavity metal-dielectric Fabry-Perot filters exhibit increased losses with decrease of the pass-band bandwidth due to the losses in the metal.
In multiple-cavity, metal-dielectric Fabry-Perot filters, this problem is usually solved by an induced-transmission design. This phenomenon serves as the basis of such filters so that it is possible to match metal layers and dielectric spacer thicknesses such that, for a given wavelength and angle of incidence, the localization of the light in the metal layers during transmission is minimal at the same time it is maximized inside the dielectric layers. Using such a design, it is still usually not practical or possible to achieve perfect transmission. However, the transmission can be made somewhat greater than 50%, combined with near square pass band shape and simultaneously good control of rejection and pass band bandwidth. Multiple-cavity metal-dielectric filters can, however, have some significant disadvantages. In addition to an angular shift of the wavelength position of the pass band due to absorption in metal layer, such filters may not be suitable for high-power applications. The temperature dependence of the optical performance of such filters also can be the strongest among all interference-based filters.
Various different designs for spectral filters are known. Among them it worthwhile to mention ultraviolet optical filter disclosed in Lehmann et al., Appl. Phys. Lett. V 78, N.5, January 2001. The filter configuration of Lehmann et al., Appl. Phys. Lett. V 78, N.5, January 2001 is based on the spectral filtering of light in an array of leaky waveguides in the form of pores in Macroporous Silicon (“MPSi”). One such an illustrative method of optical filter manufacturing consists of forming a freestanding macropore array from N-doped Si wafer in fluoride-containing electrolyte under certain backside illumination conditions. Precise control over the pore distribution across the surface of the wafer may be possible if preliminary patterning of the silicon wafer surface with regularly distributed depressions (so-called “etch pits”) is performed. A method of manufacturing such filters by forming of free-standing macropore arrays from n-doped Si wafer is disclosed and claimed can be found for example in U.S. Pat. No. 5,262,021 issued to V. Lehmann et al. Nov. 16, 1993. Lehmann also discloses the use of such arrays as optical filters. However, it appears that the method of removing the macroporous layer from the Si wafer, as disclosed in U.S. Pat. No. 5,262,021, will result in the second surface of the macroporous layer being inherently rough, causing higher losses due to scattering. Lehmann seems to use the MPSi layer without any further modifications. Thus, while such filters exhibit some short-pass filtering, the transmission spectral shape through them may be unusable for commercial applications due to the wide blocking edge.
Macroporous silicon layers with modulated pore diameters throughout the pore depth are disclosed in, for example, U.S. Pat. No. 5,987,208 issued to U. Gruning and V. Lehmann et al. Nov. 16, 1999 or J. Schilling et al., Appl. Phys. Lett. V 78, N.9, February 2001. These structures may not exhibit advantageous properties such as independence of the spectral response of the filter on the angle of light incidence for at least two reasons. First, the structure of these filters (i.e. a hexagonal array of pores) may not be suitable to act as an array of waveguides, so the filtering may be directly affected by the angle of incidence of light on the structure. Second, the pore modulation period, pore array period and the Bragg wavelength seem to be chosen in the prior art so that the light traveling through such a structure effectively averages the dielectric properties of the structure (similar to what happens in microporous silicon-based filters). The resulting optical behavior will therefore likely be close to that of ordinary interference filters.
FIG. 1 is a diagrammatic perspective view of an exemplary prior art freestanding MPSi uniform pore array section of a uniform cubic lattice. The FIG. 1 exemplary prior art spectral filter consists of air- or vacuum-filled macropores 1.2 disposed in the silicon host wafer 1.1 starting from the 1st surface 1.3 of the filter wafer and ending at the 2nd surface 1.4 of the filter wafer. The macropores 1.2 are disposed such that an ordered, uniform macropore array is formed (the ordering may be a key attribute). The pore ends are open at both first and second surfaces of the silicon wafer 1.1. Since silicon is opaque in the deep UV, UV, visible and part of the near IR wavelength ranges, light can pass through the structure shown in FIG. 1 only through the pores. As shown in FIG. 2, the silicon absorption coefficient k is very high at wavelengths below ˜400 nm and moderately high at wavelengths below ˜900 nm, which blocks all radiation coming through the silicon having a thickness of about 50 μm or more.
Since pore diameters of 100 nm to 5000 nm are comparable to the wavelength of light (200 nm-1000 nm) and, due to the high aspect ratios possible in MPSi structures ((t/d)>30), the transmission through such a macroporous structure at wavelengths below about 700 nm takes place through leaky waveguide modes. In such leaky waveguides, the cores are air or vacuum-filled, while the reflective walls are the pore walls. Hence, MPSi material can be considered as an ordered array of leaky waveguides at wavelengths below about 700 nm. By means of the high absorption of the walls, each leaky waveguide pore can be considered to be independent of the others in the visible, UV and deep UV spectral ranges if they are separated by silicon walls with thicknesses >20-100 nm.
In the near IR and IR wavelength ranges, the nature of the transmission through the filter of FIG. 1 changes. This happens because silicon becomes less opaque at 700-900 nm and becomes transparent at wavelengths starting approximately from 1100 nm. Light at these wavelengths can pass through the MPSi structure of FIG. 1 not only through the pores, but also through the silicon host. Due to the porous nature of the silicon host, the transmission of light propagating at angles close to the perpendicular (normal) direction to the surface of the MPSi structure occurs through waveguide modes confined in the silicon host for wavelengths comparable to the pitch of the pore array. As a high refractive index material, silicon can support waveguide modes if surrounded by a lower refractive index material (air or vacuum).
Since close packing of the pores is essential for efficient transmission through the filter of FIG. 1, such a structure can be considered to some approximation in the near IR and IR wavelength ranges as an array of Si waveguides in an air host. For the light propagating at oblique angles (>about ±15° from the normal), non-waveguiding channels of transmission through the structures of FIG. 1 arise. However, such transmission is accomplished by strong reflection and scattering because of the necessity of crossing the boundaries between host and pore, and in the far field of the filter in many cases this light can be neglected. In the near field, however, such transmission channels usually should be taken into account. When the wavelength of light becomes much larger than the pore array pitch, the light starts interacting with the MPSi layer as if it were a single layer of uniform material having its dielectric constants averaged throughout the pores and the host. As an illustration, for a square array of pores with 4 μm pitch, transmission as through a layer of uniform material takes place starting approximately at a wavelength of 20 μm.
Depending on pore size and pore array geometry, leaky waveguides in the deep UV, UV, and visible spectral ranges and waveguides in the near IR and IR spectral ranges can be either single mode (i.e., supporting only the fundamental mode) or multimode (higher order modes are also supported).
For both leaky waveguide and waveguide modes, the mode-coupling coefficient is the highest for the fundamental mode and quickly decreases with the increasing mode order. If a plane-parallel beam of light is incident on the MPSi interface, the coupling efficiency to the leaky waveguide fundamental mode can be roughly estimated as:             P      ⁡              (        λ        )              ≈          S              S        uc              ,where S is the area of pores 1.2 in FIG. 1, while Suc is the area of a MPSi array unit cell (which can be introduced for ordered MPSi arrays only). In other words, to a good approximation, P00LW(λ)˜p in the UV spectral range, where p is the porosity of an MPSi filter near the first MPSi wafer interface. For the waveguide transmission (i.e., for Near IR or IR wavelength ranges), the formula for the coupling efficiency, P00W(λ), can also be simplified to:                     P        00        W            ⁡              (        λ        )              ≈                            4          ⁢                                                    n                Si                            ⁡                              (                λ                )                                      ·                          n              I                                                            (                                                            n                  Si                                ⁡                                  (                  λ                  )                                            +                              n                I                                      )                    2                    ·                                    S            uc                    -                      S            p                                    S          uc                      ,where nSi(λ) is the refractive index of silicon at the wavelength λ and n1 is the refractive index of the medium from where light is incident on MPSi layer. For the most common case of air             P      00      W        ⁡          (      λ      )        ≈                    4        ⁢                              n            Si                    ⁡                      (            λ            )                                                (                                                    n                Si                            ⁡                              (                λ                )                                      +            1                    )                2              ·                                        S            uc                    -                      S            p                                    S          uc                    .      In other words, P0.0LW(λ)≈p and P0.0W(λ)≈0.69(1−p), where p is porosity of MPSi layer. For the filter of FIG. 1, the approximation given above for waveguide case (i.e., for near IR and IR wavelength ranges) is not as good as for the leaky waveguide case (deep UV, UV and VIS spectral ranges) due to strong cross-coupling between neighbor waveguides and due to the presence of the previously described non-waveguiding channel of transmission. This cross-coupling is not taken into account by the approximation set forth above.
At the second interface of MPSi filter, the light from waveguide ends (leaky or not, as applicable) is emitted with a divergence governed by the numerical aperture, NA, and wavelength. In the far field, the destructive and constructive interference of all light sources in the form of leaky waveguide or waveguide ends takes place. In the case of an ordered MPSi array, this leads to a number of diffraction orders, which are defined by the pore array geometry (i.e., by the relationship between pore size, and pore-to-pore distance) and the wavelength of the light. For most applications of optical filters, only light outcoupled into the 0th-diffraction order is of interest. However, some applications are not sensitive to the outcoupling of light to higher diffraction orders, for instance when the filter is directly mounted on the top of a photodetector or a detector array. In other cases, the main source of outcoupling losses is the redistribution of light into higher diffraction orders. Such losses are sensitive to both wavelength and pore array geometry. They are more pronounced at short wavelengths due to the higher number of diffraction orders.
Outcoupling losses can be completely suppressed for any given wavelength if the MPSi array period is less than or equal to that wavelength. For instance, for a 1550 nm wavelength that is important for optical communications, this will require a pore array period on the order of 1550 nm or less and pore diameters of about 300-1000 nm.
The exemplary prior art spectral filter structure of FIG. 1 cannot be used as a band-pass or narrow band-pass filter in the near IR or IR since the structure of FIG. 1 passes the light above the absorption band of silicon uniformly and does not offer any means to select a specific passing or blocking band. In order for it to serve as a band-pass or narrow bandpass filter, some improvements in its design should be made.
Information about manufacturing of straight pore MPSi arrays can be found in U.S. Pat. No. 5,262,021 issued to V. Lehmann et al. Nov. 16, 1993 (which claims priority to Fed. Rep. Of Germany Patent # 4202454, issued Jan. 29, 1992), in which a method of the formation of free-standing macropore arrays from an n-doped Si wafer is disclosed. Lehmann also describes the use of such arrays as optical filters. However, as explained above, Lehmann's structures cannot provide any useful filtering in the IR spectral range. In addition, the method of removing the macroporous layer from the Si wafer, as disclosed in U.S. Pat. No. 5,262,021, will result in the second surface of the macroporous layer being inherently rough, causing high losses due to scattering.
Macroporous silicon layers with modulated pore diameters throughout the pore depth is disclosed in, for example, [U.S. Pat. No. 5,987,208 issued to U. Gruning and V. Lehmann et al. Nov. 16, 1999] or [J. Schilling et al., Appl. Phys. Lett. V 78, N.9, February 2001]. However, in the first of these disclosures only a 3-layer system is disclosed which is not useful for spectral filtering (it was designed to be a lateral waveguiding structure), while in second disclosure the dimensions of the MPSi array are not suitable for providing omnidirectional filtering. The silicon island waveguides are strongly coupled due to the disclosed pore topology (the structure has been designed to provide Photonic Bandgap properties, not an array of uncoupled waveguides).
There are also several disclosures related to the method of manufacture of macroporous structures with controlled positions of the pores. One of the first such disclosures is U.S. Pat. No. 4,874,484 issued to H. Föll and V. Lehmann issued Oct. 17, 1989 (which claims priority to Fed. Rep. Of Germany Patent # 3717851 dated May 27, 1987). This patent describes a method of generating MPSi arrays from n-doped (100)-oriented silicon wafers in HF-based aqueous electrolytes (i.e., electrolytes based on HF diluted with water) under the presence of backside illumination. It also describes a method of controlling the position of macropores through formation of etch-pits. Etch pits are typically, but not exclusively, pyramid-shaped openings formed on the silicon or other semiconductor surface that can be formed through mask openings upon exposure to anisotropic chemical etchants. In addition, the use of wetting agents (such as formaldehyde) and controlling the pore profile through chronologically-varying applied electrical potentials also were disclosed. However, the geometries of array provided in this disclosure can not satisfy the requirement of low optical coupling between neighboring silicon island waveguides required for omnidirectional filtering. Thus, the optical filtering applications of the structures disclosed by Föll and Lehmann were not foreseen, and the method of fabrication disclosed therein is not suitable for the formation of a deep MPSi layer with controllable pore diameters along the entire depth of the MPSi layer. This is because changes of the pore growth speed with depth are not taken into account.
A method of MPSi layer formation in non-aqueous electrolytes is disclosed in U.S. Pat. No. 5,348,627 issued Sep. 20, 1994 and U.S. Pat. No. 5,431,766 issued Jul. 11, 1995, both to E. K. Propst and P. A. Kohl. Organic solvent-based electrolytes were used for forming porous layers in n-doped silicon under the influence the front-side illumination. Example solvent based electrolytes are acetonitrile (MeCN), diemethyl formamide (DMF), propylene carbonate (C3O3H6) and methylene chloride (CH2Cl2)) containing organic supporting electrolytes, such as the examples of tetrabutilammonium perchlorate (C16H36NClO4), tetramethylammonium perchlorate (C4H12NClO4) and anhydrous sources of fluoride, HF, fluoroborate (BF4−), tetrabutylammonium tetrafluoroborate (TBAFB), aluminum hexafluorate (AlF63−) and hydrogen difluoride (HF2−). However, the MPSi layer quality obtained by using this method is of generally poor optical quality with strong pore wall erosion and branching.
A method of manufacturing ordered free-standing MPSi arrays, including pore walls coated by a semiconducting layer with follow-on oxidizing or nitriding through a CVD process was disclosed in U.S. Pat. No. 5,544,772 issued Aug. 13, 1996 to R. J. Soave, et al., in relation to the production of microchannel plate electron multipliers. N-doped silicon wafers, photoelectrochemically etched in an HF-based aqueous electrolyte, were disclosed. Another method of manufacturing MPSi-based microchannel plate electron multipliers is disclosed in U.S. Pat. No. 5,997,713 issued Dec. 7, 1999 to C. P. Beetz et al. This patent describes an ordered, freestanding MPSi array made by the electrochemical etching of a p-doped silicon wafer. Both aqueous and non-aqueous (e.g., acetonitrile, tetrabuthylsulfoxide, propylene carbonate or metholene chloride-based) electrolytes based on both HF and fluoride salts were disclosed for MPSi layer manufacturing. Covering the pore walls of a freestanding MPSi array with a dynode and insulating materials through CVD, sol-gel coating, electrolytic deposition, electrodeposition and electroless plating were disclosed. The use of mechanical grinding, polishing, plasma etching or chemical back-thinning to remove the remaining part of the silicon wafer in line with the pores were disclosed. The use of a surfactant to improve pore quality was also taught. However, such methods are related to just a straight pore MPSi arrays, thus not suitable for IR spectral filtering.
The use of a conductivity-promoting agent in organic-based electrolytes (e.g., DMF) during the photoelectrochemical etching of n-doped silicon was disclosed in S. Izuo et al., Sensors and Actuators A 97-98 (2002), pp. 720-724. The use of isopropanol ((CH3)2CHOH) as a basis for an organic electrolyte for electrochemical etching of p-doped silicon was disclosed in, for example, A. Vyatkin et al., J. of the Electrochem. Soc., 149 (1), 2002, pp. G70-G76. The use of ethanol (C2H5OH) to reduce hydrogen bubble formation during electrochemical etching of silicon as an addition to aqueous HF-based electrolytes was disclosed in, for example, K. Barla et al., J. Cryst. Growth, 68, p. 721 (1984). Completely filling the pores with silicon dioxide or doped silicon dioxide through CVD, particularly to create optical waveguides (similar to optical fibers in structure) for integrated circuit interconnects was disclosed in U.S. Pat. No. 6,526,191 B1 issued Feb. 25, 2003 to Geusic et al. A detailed review of the various aspects of MPSi formation can be found in H. Föll et. al, Mat. Sci. Eng. R 39 (2002), pp. 93-141.
In addition to silicon, macropores have been obtained in other types of semiconductor and ceramic materials. Macropores obtained in n-type GaAs by electrochemical etching in acidic electrolytes (aqueous HCl-based) were reported by, for example, D. J. Lockwood et al., Physica E, 4, p. 102 (1999) and S. Langa et al., Appl. Phys. Lett. 78(8), pp. 1074-1076, (2001). Macropores obtained in n-type GaP by electrochemical etching were reported by B. H. Erne et al., Adv. Mater., 7, p. 739 (1995). Macropore formation during the electrochemical etching of n-type InP (in aqueous and organic solutions of HCl and mixtures of HCl and H2SO4) was reported by P. A. Kohl et al., J. Electrochem. Soc., 130, p. 228 (1983) and more recently by Schmuki P et al., Physica Status Solidi A, 182 (1), pp. 51-61, (2000); S. Langa et al., J. Electrochem. Soc. Lett., 3 (11), p. 514, (2000). Macroporous GaN formation during electrochemical etching was reported by J. v. d. Lagemaat, Utrecht (1998). Macropore formation during electrochemical etching of Ge was reported by S. Langa et al., Phys. Stat. Sol. (A), 195 (3), R4-R6 (2003). Reviews of macropore formation in III-V semiconductors can be found in H. Föll et al., Phys. Stat. Sol. A, 197 (1), p. 64, (2003); M. Christophersen et al., Phys. Stat. Sol. A, 197 (1), p. 197, (2003), and H. Föll et al., Adv. Materials, Review, 2003, 15, pp. 183-198, (2003).
Spectral filter technology have not yet been practically demonstrated in any porous semiconductor material other than silicon. Ordered pore arrays were reported for n-doped InP (S. Langa et al., Phys. Stat. Sol A, 197 (1), p. 77, (2003)), but in that context the order which was obtained was due to self-organization rather than due to pore formation in predetermined locations. No post-growth coating of the pore walls was disclosed, nor was intentional pore cross-section modulation investigated.
In addition to electrochemical etching, other methods of producing pore-like structures are known to those skilled in the art. As an example, deep Reactive Ion Etching (DRIE) has been used to produce relatively high aspect ratio hole structures with CVD-deposited diamond coated walls for microchannel plate electron multipliers (see, for example, U.S. Pat. No. 6,521,149 issued Feb. 18, 2003 to Mearini et al.). Such structures are also made freestanding by backside removal of the silicon through grinding, polishing or etching. Various methods of filling high vertical aspect ratio structures (used in integrated circuit chip manufacturing) by various materials can be found in U.S. Pat. No. 5,645,684 issued Jul. 8, 1997 to C. G. Keller.
Exemplary New Spectral Filter Designs
We provide in one non-limiting illustrative exemplary arrangement, an improved near IR, mid IR or far IR filter configuration based on a substantially uniform array of waveguides made of porous semiconductor (where pores are straight and non-branching). Further, the pore cross sections are modulated at least along part of their depths while other parts may be left unmodulated, or the entire depths can be modulated.
Such spectral optical filters can be used for band-pass, narrow-band pass or band blocking spectral filtering, and provide significant advantages. Exemplary advantages of particular implementations include, but are not limited to:                Omnidirectionality, i.e., absence of spectral shape dependence of transmission (for transmission type optical filters) or reflection (for reflection type optical filters) spectra on the angle of light incidence within the acceptance angles of the filter. Manufacturability (i.e., the ability to fabricate such filters relatively simply and inexpensively compared to the other filter configurations known by those skilled in the art).        The absence of material delamination problems as found with multilayer interference filters.        
Exemplary non-limiting configurations are based on the formation of a large number of identical, mutually de-coupled waveguides arranged with respect to each other such that the transmission through the array is possible mainly through at least one of the waveguide modes of the assembly of waveguides. The transmission or reflection spectrum of each of said waveguides is wavelength dependent due to modulation of the waveguide cross-sections and can be tuned to the desired spectral shape and position by modifying the structure of said pore cross-section modulation. In addition, one or both broad faces of the filter containing the waveguide ends can be covered by an antireflective coating such as, for example, a single layer of transparent dielectric material of quarter-wavelength thickness and refractive index close to the square root of the refractive index of said waveguide material or, alternately covered by dielectric multilayer coatings. These coatings, covering the broad faces of the non-pore material forming the waveguide ends, suppress the coupling and/or outcoupling losses, thus increasing the transmittance of such filters within the pass band of said filters.
A waveguide array can be formed in a semiconductor wafer in the form of channels going through the wafer (semiconductor islands between the pores). Such a structure can be fabricated, for example, by forming the porous semiconductor layer by means of electrochemical or photoelectrochemical etching of a single crystal semiconductor wafer as deeply as necessary (waveguide lengths chose by design). By this procedure, a porous semiconductor layer is made with the pores extending at least partially through the semiconductor. Semiconductor islands between pores formed by such a process will serve as waveguides at wavelengths within the transparency range of said semiconductor, while the pores will reduce the coupling between the waveguides. The modulation of the cross sections of the waveguides can be achieved through modulating the pore diameters along their depths by modulating the electrochemical etching parameters during the electrochemical etching process.
For example, the parameters available for modulation include the current density, illumination intensity or other parameters known to those skilled in the art. Said semiconductor material can be silicon (p-type doped or n-type doped), gallium arsenide, indium phosphide, or any other material that can be shown to form straight pores during electrochemical etching in a suitable electrolyte and under suitable conditions. In order to suppress the cross coupling between the waveguides and to reinforce the structure mechanically, the pore walls can be coated by a layer of material that is substantially transparent within the pass band of the filter and that has a lower refractive index than said semiconductor. Such a layer may serve as a waveguide cladding. The covering of the pore walls can be achieved by partial thermal oxidation of a semiconductor (principally silicon), or by depositing a dielectric single layer or multilayer onto the pore walls by Chemical Vapor Deposition or by any other deposition, sputtering, evaporation or growth process known to those skilled in the art. Covering the substrate or wafer surface (or surfaces) between the pores by an antireflective structure can be accomplished by directional deposition techniques, such as physical vapor deposition, magnetron sputtering, thermal or electron beam evaporation, ion assisted ion plating or any other technique known to those skilled in the art. If the filter structure is too fragile for its intended use, the porous layer can be reinforced by sealing between two plates of a material that is transparent over the transparency wavelength range of the porous filter or a wider range. Such plates can be, for instance, of glass, silica, or any other transparent dielectric known to those skilled in the art.
In order to suppress the transmission in the porous layer through channels other than the waveguiding channels of transmission, the pore walls can be coated by a layer of absorptive or reflective material, at least over some length along the pores. More over, the pores can be completely filled by an absorptive and/or reflective material or materials. Metal can serve as a nonlimiting example of such an absorptive or reflective material. In such a spectral filter configuration, the use of the transparent cladding layer disclosed in the previous paragraph is strongly desired to minimize the waveguide losses associated with the introduction of said absorptive or reflective layer onto the pore walls. Said pore wall coating by an absorptive and/or reflective layer can be accomplished by chemical vapor deposition, atomic layer deposition or any other method known to those skilled in the art. Said complete filling of the pores with an absorptive and/or reflective material may be accomplished by chemical vapor deposition, injection molding, dye casting, capillary absorption of a liquid into the pores or by any other method known to those skilled in the art.
The pores can be disposed across the broad surfaces of the wafer or substrate with a predetermined pattern having a predetermined simple symmetry (for example, cubic or hexagonal). Alternatively, said pores can be disposed into a pattern having more complex symmetry. Additionally, the pores may have circular or near-square cross-sections.
In one exemplary illustrative non-limiting implementation, said pores (and through that the silicon island waveguides) can be made to have tapered ends at least at one (first or second) surface of said filter, or to taper uniformly or non-uniformly along their entire lengths. At the narrow end of the taper, the pore lateral cross-section can be gradually decreased when approaching the near surface (thus the waveguide lateral cross-section is gradually increased) of the filter substrate in order to increase the coupling and/or outcoupling efficiency to improve the transmittance through the filter.
The far IR spectral range can be quite important for many applications such as the nonlimiting examples of astronomy and chemical analyses. Silicon, Ge, III-V compound semiconductors or other materials known to permit ordered pore array formation through electro-chemical etching, however, are not transparent over the whole spectral range of interest. Hence, some modifications of the spectral filter design may be made to serve these applications. According to another exemplary illustrative non-limiting implementation, an improved IR filter configuration is based on a substantially uniform array of waveguides made of free-standing porous semiconductor in which the pores are straight and nonbranching. Pore cross sections are either modulated at least along part of the depths while other parts are left unmodulated, or the entire depths can be modulated. The pores are filled with the material that is transparent within the spectral range of interest (nonlimiting examples of such materials include ZnSe, CdTe and thallium iodide). The pore walls may be covered by at least one layer of transparent material different from that filling the pores completely, and having smaller refractive index, prior to said filling of the pores. In this exemplary illustrative implementation, the filled pores will act as a waveguides. The material completely filling the pores acts as a waveguide core, while the material covering pore walls (if any) serves as a waveguide cladding. The porous semiconductor matrix can be oxidized before filling the pores to reduce its refractive index and through that reduce the cross coupling between neighboring waveguides. Unlike the previously described exemplary illustrative implementation, the ordering of the pore array (and through that of the waveguide array) is not strictly required. Only uniformity of the pore sizes is needed. However, ordering still can be an advantageous feature.
According to a further exemplary illustrative non-limiting implementation, the first, the second or both surfaces of said filter wafer may be coated with an antireflective structure after said pore filling to suppress the coupling and outcoupling losses. Said antireflective coating can be a single layer antireflective coating, or, alternatively, can be made in the form of a multilayer antireflective coating. The antireflective coating can be deposited through chemical or physical vapor deposition or by any other technique known to those skilled in the art. Said pore filling can be accomplished by chemical vapor deposition, injection molding, dye casting, capillary absorption of a liquid into the pores or by any other method known to those skilled in the art.
The resulting exemplary non-limiting illustrative filters can have the advantages of stability. They do not exhibit vulnerability to delamination of the different materials and offer remarkable stability over wide range of temperatures and large temperature gradients. They also offer transmittance comparable to that of prior art narrow bandpass, bandpass and band blocking filters combined with omnidirectionality, i.e. independence of the spectral position of the filtering feature (e.g., reflection band, transmission valley or transmission edge) on the angle of incidence. Such filters are useful for a wide variety of applications, including applications where currently available filter systems cannot provide acceptable performance (e.g., a variety of analytical devices, wavelength division multiplexing, astronomical instrumentation, spectroscopy, etc.).
This specification also discloses exemplary non-limiting illustrative methods for manufacturing spectral filters. According to one implementation, spectral filters can be produced by:    preparing the semiconductor wafer having first and second surfaces wherein said first surface is substantially flat, and    anodically etching the substrate wafer to produce a structured layer having pores with controlled depths defined at least partially therethrough, said pores having coherently modulated cross-sections at least over a part of said depth.
The porous layer can be formed through electrochemical etching of said semiconductor wafer in acidic electrolyte. The etching method may include connecting the substrate as an electrode, contacting the first surface of the substrate with an electrolyte, setting a current density (or voltage, depending on the type of semiconductor material used and type of doping of said semiconductor material used) that will influence etching erosion, and continuing the etching to form said pores extending to a desired depth substantially perpendicularly to said first surface. Said semiconductor wafer can be, but is not limited to, a silicon wafer. Preliminary depressions can be formed on the first surface of said wafer (etch pits) to control the locations of the pores to be formed in the electrochemical etching process. Said etch pits can be formed through the application of a photoresist layer on the first surface of the semiconductor wafer, photolithographically defying the pattern of openings and chemically or reactive ion etching the etch pits through said openings. Alternatively, said etch pits can be formed by depositing a material layer with different chemical properties than that of the substrate by means of chemical or physical vapor deposition, thermal oxidation, epitaxial growth, sol-gel coating or any other technique known to those skilled in the art. A further step may be the application of a photoresist layer on the top of said material, photolithographically defining the pattern of openings in the photoresist layer, transferring said patterns into said layer through chemical or reactive ion etching and transforming the resultant pattern into a corresponding etch pit pattern through chemical or reactive ion etching. Said layer of material with different chemical properties than that of the substrate wafer may then be removed through chemical etching, reactive ion etching or any other method known to those skilled in the art, or may not be removed.
More specifically, said semiconductor wafer can be an n-doped, (100) orientated silicon wafer. The electrolyte can be in this case an HF-based aqueous acidic electrolyte. Alternatively, the electrolyte can be an HF-based organic electrolyte. Alternatively, said semiconductor wafer can be a p-doped, <100> oriented silicon wafer. The electrolyte in this case may be HF-based organic electrolyte. The electrolyte may contain hydrofluoric acid in a range of 0.5% to 50%, but preferably 2 to 10% by volume. A second surface of the substrate wafer that lies opposite the first surface may be illuminated during electrochemical etching. The electrolyte may additionally contain an oxidation agent, a hydrogen reducing agent (e.g., selected from the group of chemicals consisting of mono functional alkyl alcohols, tri functional alkyl alcohols), a viscosity increasing agent, a conductivity-modifying agent, and/or other organic and inorganic additives. One or more of the electrochemical process parameters such as current density (for p-doped Si wafers), applied voltage, electrolyte temperature and/or illumination intensity (for n-doped Si wafers) can vary in a predetermined fashion during the pore growth process to provide the pores with needed variations in cross section. As a further alternative, said semiconductor wafer can be of material chosen from the full possible range of alloys and compounds of zinc, cadmium, mercury, silicon, germanium, tin, lead, aluminum, gallium, indium, bismuth, nitrogen, oxygen, phosphorus, arsenic, antimony, sulfur, selenium and tellurium. The electrolyte may be an acidic electrolyte with the acid suitably chosen for pore formation in the particular semiconductor material.
According to another non-limiting illustrative arrangement, the completed porous structure is sealed from the porous side by a plate transparent in the spectral range of interest. Further, according to another aspect of the same arrangement, at least one of the first or second surfaces of said semiconductor wafer containing a porous layer is coated by an antireflective structure.
According to a further illustrative non-limiting method of manufacturing a spectral filter, the filter can be produced by:    preparing a semiconductor wafer having first and second surfaces wherein said first surface is substantially flat, and    anodically etching the substrate wafer to produce a structured layer having pores with controlled depths defined at least partially therethrough, said pores having coherently modulated cross-sections at least over a part of said depth,    removing the un-etched part of said wafer at the ends of the pores, and    coating the pore walls with at least one layer of transparent material.
The first and the second steps of this illustrative non-limiting method of manufacturing a spectral filter are the same as those of the previous illustrative method described above.
Said removal of the unetched part of the wafer can be performed through grinding, polishing, chemical-mechanical polishing, chemical etching, reactive ion etching or any other method known to those skilled in the art.
Said coating of the pore walls with at least one layer of transparent material can be accomplished through chemical vapor deposition, thermal oxidation, liquid immersion or any other method known to those skilled in the art.
According to another non-limiting exemplary illustrative arrangement, the porous structure so obtained is sealed between two transparent plates. At least one surface of the porous layer can be coated by an antireflective structure.
A further exemplary illustrative non-limiting method of manufacturing a spectral filter can be accomplished by:    preparing the semiconductor wafer having first and second surfaces wherein said first surface is substantially flat, and    anodically etching the substrate wafer to produce a structured layer having pores with controlled depths defined at least partially therethrough, said pores having coherently modulated cross-sections at least over a part of said depth,    removing the un-etched part of said wafer at the ends of the pores, DON'T WE coating the pore walls with at least one layer of transparent material, and    coating the pore walls with at least one layer of absorptive and/or reflective material.
The first, second, third and fourth steps of this illustrative non-limiting method of manufacturing a spectral filter are the same as those of the previous illustrative method described above. Said coating of the pore walls with at least one layer of absorptive and/or reflective material may be performed through a chemical vapor deposition technique as one non-limiting example. Alternatively, the pores can be completely filled by said absorptive and/or reflective material. Said pore filling can be accomplished by chemical vapor deposition, injection molding, dye casting, capillary absorption of a liquid into the pores or by any other method known to those skilled in the art.
A further exemplary illustrative non-limiting method of manufacturing a spectral filter can be accomplished by:    preparing a semiconductor wafer having first and second surfaces wherein said first surface is substantially flat, and    anodically etching the substrate wafer to produce a structured layer having pores with controlled depths defined at least partially therethrough, said pores having coherently modulated cross-sections at least over a part of said depth,    removing the un-etched part of said wafer at the ends of the pores,    Thermally oxidizing said porous layer coating the pore walls with at least one layer of transparent material serving as a waveguide cladding, and    filling the pore walls with at least one layer of transparent material serving as a waveguide core.
The first, second, third and fifth steps of this illustrative non-limiting method of manufacturing a spectral filter are the same as those of the previous illustrative method described above. Said thermal oxidation may be performed by placing the porous layer into an oxidation furnace with a temperature in the range of 900 to 1400° C. in an ambient atmosphere of dry oxygen atmosphere or wet oxygen. Said filling of the pore walls with at least one layer serving as a waveguide core can be performed through a chemical vapor deposition technique as one non-limiting example.